1. FIELD OF THE INVENTION
The present invention relates to a method for producing a relief pattern.
2. DESCRIPTION OF THE PRIOR ART
Heretofore, a relief pattern of glass or a semiconductor has been made by a process where a photoresist layer coated on a glass or semiconductor support is image-wise exposed to light and developed to uncover the surface of the support at the exposed or unexposed areas, the support at uncovered areas is then etched, and then the remaining resist at non-exposed or exposed areas is removed to obtain a relief pattern of the support. The above described process is called photoetching. Relief patterns obtained by photoetching possess a uniform height (thickness).
However, some uses require a relief having a height which gradually (continuously) varies. For example, an optical guide which is used as an element for an optical IC (Integrated Circuit) usually has a uniform rectangular section; however it is preferred that one end of the guide be tapered when the guide is connected to other elements at that end. It is difficult to form such a tapered relief pattern using conventional photoetching.
Further, a phase hologram of the relief type is desired to have a continuously changing relief. However, in a relief type phase hologram obtained by conventional photoetching it is difficult to change the height of the relief in a continuous manner, i.e., photoresists reproduce continuous gradation only with difficulty, accordingly when a hologram is formed using photoetching, the cross-section of the relief obtained varies in an abrupt, discontinuous fashion. Therefore, the physical/optical characteristics of a relief type phase hologram obtained by photoetching have been unsatisfactory.
Moreover, conventional photoetching has the disadvantage that the light-sensitivity of photoresists is low. In addition, conventional photoetching requires chemical processes such as coating of a photoresist, developing, chemical etching, removal of photoresist, and, accordingly, conventional photoetching results in environmental pollution through the discharge of used chemicals.
Heretofore, emulsion holograms (amplitude holograms obtained by exposing and developing a silver halide emulsion layer formed on a support, and phase holograms obtained by bleaching such an amplitude holograms) have been used as amplitude holograms in which an image is recorded as black and white stripes (black stripes comprise silver grains) and as phase holograms obtained by bleaching such amplitude holograms. Since amplitude holograms have a low diffraction efficiency, the silver grains are often bleached and converted into silver halides or other transparent silver compounds to obtain a so-called emulsion phase hologram having a higher diffraction efficiency.
However, such emulsion phase holograms have the defect that the silver compound in the hologram is colored by print-out during use, and the diffraction efficiency thereof decreases, i.e., the light resistance of the hologram is low.
Further, emulsion phase holograms have the defect that since the binder of the holograms is an organic material, such as gelatin, the heat resistance is low, i.e., water-soluble polymers which can be used as binders for photographic emulsions color when heated to about 150.degree. C, therefore it has been difficult to use emulsion holograms at a temperature higher than about 150.degree. C.
Heretofore, it has been known to produce relief type phase hologram by frost xerography using a thermoplastic polymer. However, the apparatus to produce such a relief type phase hologram is complicated, and the phase hologram obtained has poor light, heat and abrasion resistance, and further has the defect that the physical/optical characteristics are poor since a relief having a continuous gradation is not obtained.